Adding a stress_experimental suite and moving a test to it.

This suite is meant to verify tests before they are promoted to the
actual stress suite.

Moved platform_ExternalUsbPeripherals test to the new suite.

BUG=None
TEST=None

Change-Id: Ie2733044eb0a2e144dc546b2982bf19d91f6d459
Reviewed-on: https://chromium-review.googlesource.com/223753
Reviewed-by: Kalin Stoyanov <kalin@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Yusuf Mohsinally <mohsinally@chromium.org>
Tested-by: Yusuf Mohsinally <mohsinally@chromium.org>
3 files changed